templates <int T> C++

Question

What does it mean to define a template with template <int N>, I read this from an example in a book:

typedef map<int, double> row;
template <int N> struct matrix;
{
   map <int, row> my_matrix;
}

I have usually seen templates which are followed by class and then type, i.e. template <class int>

Answer 1

Template parameters may be of any of the three kinds of C++ entities: values, types, or templates:

template <int N> struct Foo;                        // N is a value

template <typename T> struct Bar;                   // T is a type

template <template <typename> class X> struct Zip;  // X is a template

Example usage:

Foo<10> a;

Bar<int> b;

Zip<Bar> c;

Note that this corresponds to the three ways of disamiguating dependent names:

X::a = 10;              // X::a is a value
typename X::B n = 10;   // X::B is a type
X::template C<int> m;   // X::C is a template

---

An interesting combination arises when you want to "typify" a value. This can be done like so:

template <typename T, T Val> struct Typificate;

Now you can say e.g. Typificate<int, 10> to get a unique type that represents the integer 10. This can occasionally be very useful, e.g. when T is a member function pointer (e.g. in this delegate implementation); or it can be used to create value-unique overloads for tag dispatch, etc.